Move plugins to manifest, pin Docker version, add Makefile

- Add plugins.txt listing all plugins for reproducible installs
- Add Makefile with setup/start/stop/install-plugins targets
- Remove user/plugins/ from git tracking
- Pin Docker image to 1.7.49.5-ls244

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

plugins/problems/scss/_breadcrumbs.scss

@@ -1,29 +0,0 @@
-// Breadcrumbs
-.breadcrumb {
-  list-style: none;
-  margin: $unit-1 0;
-  padding: $unit-1 0;
-
-  .breadcrumb-item {
-    color: $gray-color-dark;
-    display: inline-block;
-    margin: 0;
-    padding: $unit-1 0;
-
-    &:not(:last-child) {
-      margin-right: $unit-1;
-
-      a {
-        color: $gray-color-dark;
-      }
-    }
-
-    &:not(:first-child) {
-      &::before {
-        color: $gray-color-light;
-        content: "/";
-        padding-right: $unit-2;
-      }
-    }
-  }
-}